i'd really love to be able to quickly check which creatures i own are legendary, for example...

Hello.

This is kind of an obscure feature, but if you chose Subtype from the filter dropdown, whatever you type in the textbox is searched in the entire type line of the card. So this: http://deckbox.org/sets/64395?f=47TGVnZW5kYXJ5 gets what you need.

oh sweet--i didn't realize that the subtype filter actually checked the whole type line. brilliant! i thank you!